A number of hydrocarbons, especially the lower-boiling light hydrocarbons, as found in formation fluids or natural gas are known to form hydrates in conjunction with water under a variety of conditions. This may be particularly true at lower temperatures and higher pressures.
Hydrates usually exist in agglomerated solid forms that are essentially insoluble in the fluid itself. As a result, any solids in a formation or natural gas fluid are at least a nuisance for the production, handling and transportation of these fluids. It is not uncommon for agglomerated hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices, vessels, tanks, and/or other equipment, resulting in shutdown, loss of production, risk of explosion and injury or unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, natural gas hydrates are of substantial interest as well as a concern to many industries, particularly the petroleum and natural gas industries.
Gas and hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally may be composed of crystals formed by host water molecules that surround the gas or hydrocarbon guest molecules. Without being limited to a particular understanding, the smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are sometimes more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it may be possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen, oxygen and hydrogen sulfide are known to form hydrates under the proper conditions. Several of these non-hydrocarbons, such as carbon dioxide and nitrogen, are known to exist in produced hydrocarbon fluids and therefore present an added risk of hydrate formation.
Controlling, inhibiting, and/or preventing hydrate formation, and particularly removing hydrate deposits may be a difficult, dangerous and expensive process. Presently, hydrate formation may be often controlled by using chemicals and/or active heating. Remediation of a plugged conduit often employs some combination of active heating, chemicals and/or depressurization. The use of inhibition chemicals, depressurization and/or heaters may be logistically complex and expensive and may incur a certain amount risk to field personnel.
Some arrangements are known to try to clean hydrates or other matter from wellbores using acoustic energy. The vibratory transducers used in these earlier approaches are typically operated at high vibration frequencies, in one non-limiting understanding. These high frequency vibrations are used to shatter the matrix of an already formed hydrate plug or to remove an existing deposit of hydrates or other matter. It is believed, however, that these higher frequencies are not effective in preventing the initial deposition of hydrates and other deposits within portions of a wellbore or pipeline. Thus, these prior approaches have not been effective in preventing the initial agglomeration and build-up of hydrates within the conduit.
Other systems and methods for inhibiting the deposition of natural gas hydrates are described in the parent application to this one. These techniques focused on inhibiting the formation and growth of a hydrate matrix that would allow a solid plug or blockage to develop within a flowbore. In described embodiments, an acoustic inhibitor may be associated with a wellbore proximate the wellhead and may be used to generate a low frequency acoustic energy signal that is propagated axially through the wellbore. The wellbore was used as a waveguide to propagate the energy signal. In one non-limiting embodiment, the acoustic waves are generated at a frequency in a relatively low frequency range that may be generally from about 1000 Hz to about 2200 Hz. Particularly effective frequencies for inhibiting the growth and formation of a hydrate matrix are 1130 Hz and 2000 Hz.
While existing techniques for inhibiting matrixes of hydrates and/or other deposits are useful, they are generally not sufficient to address many or most situations. Prior techniques employing sonic techniques focus their effectiveness on the inner surfaces of the pipeline or production tubing rather than on the material being transmitted through the pipeline or production tubing. As such, they do not provide any protection against hydrate deposits that might form in transportation flowbores, such as subsea pipelines.
It would be desirable if methods and apparatus were devised to make gas hydrate formation more controllable and predictable.